My Master's thesis was a review and rating of large Spanish-English dictionaries. Since then, I have conducted follow-up studies to keep up with the market.
I find the leaders in the field to be Collins (HarperCollins), Larousse, and Oxford. Each publisher appears to be trying to one-up the others with the newest and best edition. The real winner is the consumer. My joint review of these three dictionaries is found here and duplicated under both of its competitors.
Here are a few of the factors which distinguish a good bilingual dictionary from a bad one.
To begin with, ignore certain publishers' marketing ploys such as entry and translation counts. They says nothing about the value of the words chosen. Likewise, ignore the word "unabridged" in the title. No work is truly unabridged except the monumental monolingual Oxford English Dictionary.
The first valid factor to consider is lexicographic technique. A bad dictionary simply lists translations. Take, for example, the entry in the Cassell's Spanish Dictionary under the English headword loop: "lazo, gaza, nudo; ojal, presilla, alamar; anillo; recodo, comba, curva, vuelta," etc. For the English reader writing in Spanish, this is hopelessly inadequate, as the dictionary provides no clue as to which translation to use in which situation.
Compare the treatment of the same word in the far superior American Heritage Spanish Dictionary. "(length of line) lazo; (coil) vuelta; (bend) curva; (circular path) vuelta, circuito; (fastener) presilla" etc. Here, the user is given glosses in the native language to assist in identifying the right word for the context. Example sentences are also a tremendous help. The Collins, Larousse, and Oxford are all excellent in this respect, presenting a wealth of practical information to guide users through the semantic and syntactic complexities.
The second factor is organization, which is important in large desk dictionaries. In an entry for a complex word like "get," a bad dictionary may force users to lose time searching for their translation through unbroken columns that can extend for more than a page. This was a problem--now corrected--in previous versions of the large Larousse dictionary. Today, the current editions of the Larousse, Collins and Oxford divide long entries by meaning into well-titled paragraphs. This scheme makes these dictionaries a joy to use.
Third, a good dictionary should maintain an up-to-date lexicon, including such cultural and technological additions to the language as "baby sitter," "FAQ's," "hostile takeover," "software," "flash drive." Larousse, Collins and Oxford are leaders in this respect; their frequent revisions are more than mere window dressing and do a creditable job of covering the most recent additions to the language.
Fourth, idioms, slang, and cusswords can present real problems to the language learner, and a dictionary needs to handle them in a clear and frank fashion. All three dictionaries get it right, giving stylistic equivalents for translations as well as clear advice to the user.
One complaint about the Collins is that it often presents Britishisms without labeling them as such. Revisions have only partially corrected the problem. For this reason, I would not recommend this dictionary to native Spanish speakers in the US.
Oxford and Collins contain excellent "language in use" sections which give formulas for language functions such as asking for information, agreeing, disagreeing, etc., as well as formulas for letters and documents.
The bottom line on large dictionaries? Avoid Vox, Velasquez, Langenscheidt, and Cassell's. Simon & Schuster's is unsuitable as a user's only dictionary but may serve some use as part of an advanced collection. I will report on the large Harrap's when I examine it, but my opinion of their other dictionaries is quite favorable. While not perfect, Collins, Oxford, and Larousse are the best large Spanish-English dictionaries I have examined. Except as noted here, most users would be well served by any of the three.